The present invention relates to electronic devices such as thin film resonator (TFR) devices, more particularly to a method and apparatus for determining and/or improving high power reliability in these devices, and a thin film resonator (TFR) device developed from the results.
Thin film resonators (hereinafter xe2x80x9cTFRxe2x80x9d) are typically used in high-frequency, high-power environments ranging from several hundred megahertz (MHz) to several gigahertz (GHz). FIG. 1 illustrates a side view or cross-section of a typical TFR component 100. In FIG. 1, TFR component 100 includes a piezoelectric material 110 interposed between two conductive electrode layers 105 and 115, with electrode layer 115 formed on a support structure 120. The support structure 120 may be a membrane, or may be a plurality of alternating reflecting layers on a solid semiconductor substrate which may be made of silicon or quartz, for example. The piezoelectric material is preferably one selected from the group comprising at least ZnO, CdS and AlN. Electrode layers 105 and 115 are formed from a conductive material, preferably of Al, but may be formed from other conductors as well.
TFRs are often used in electronic signal filters, more particularly in TFR filter circuits applicable to a myriad of communication and microelectronic technologies. For example, TFR filter circuits may be employed in cellular, wireless and fiber-optic communications, as well as in computer or computer-related information-exchange or information-sharing systems.
The piezoelectric material in TFR resonators converts electrical to mechanical energy and vice versa, such that at its mechanical resonance frequency, the electrical behavior of the device abruptly changes. Electrical signals of particular frequencies easily pass thorough the resonators, while others will not be transmitted. These particular frequencies can typically be dictated by choosing resonator size and design. Resonators of certain sizes and design frequencies can be networked in appropriate combinations, such that they will impose desired filtering functions on signals passing through the network.
A standard approach to designing filters out of resonators is to arrange them out of simple building blocks such as in a ladder configuration alternately in a series-shunt relationship. A series element in this sense carries signal from an input toward an output, whereas a shunt element provides an alternative path for the signal to ground. The transmission or blocking characteristics of both series and shunt elements affect the final signal reaching output from input, somewhat analogous to how branching of water pipes can affect the flow through the main water line.
FIG. 2 illustrates schematically this simple building block, commonly known as a T-Cell. Referring specifically to FIG. 2, a schematic of a T-Cell building block 200 includes three TFR components 210, 220 and 230. TFR components 210 and 220 comprise the xe2x80x9cseries armxe2x80x9d portion of the T-Cell block, being connected in series between an input port 215 and an output port 225 of T-Cell 200. TFR component 230 comprises the xe2x80x9cshunt legxe2x80x9d portion of T-Cell 200, being connected in shunt between node 235 and ground. A TFR T-Cell itself may define a filter; although a TFR ladder filter typically has a plurality of these T-cells concatenated together.
Each of the shunt and series TFR components 210, 220 and 230 in the schematic T-Cell of FIG. 2 has a set of characteristic frequencies: a xe2x80x9cpolexe2x80x9d frequency and a xe2x80x9czeroxe2x80x9d frequency. The terms refer to the magnitude of the impedance to current flow through the device; impedance is low at the zero and high at the pole. The series and shunt arms in a filter typically have zero and pole frequencies slightly shifted from each other.
However, for these TFRs, as for many electronic components across the scope of microelectronic technology, one of the most serious and persistent problems has been the effects of electromigration on a component""s performance and reliability. Generally, electromigration damage is characterized by the formation of growth and voids in a metal of an electronic component. During the 1960""s, it was recognized that electromigration could cause void formation in Al interconnects of integrated circuits, leading to open circuit failure. Since then, many studies have been performed in an effort to understand the nature of electromigration, and a substantial amount of literature has been written on the subject.
A major effort has been made to solve this problem in silicon (Si) very large-scale integration technology. In fact, as on-chip device packaging densities increase, interconnect line widths must accordingly decrease; this translates into ever-higher interconnect current densities. Not only are the electromigration effects present at interconnections, but these effects are also present in any device where a film of metal (Al for example) is used as a connector or an electrode, and which has a high current density passing therethrough.
Accordingly, electromigration and its effects are likely present in the Al electrodes of many, if not all, TFR devices, particularly those used as duplex filters in wireless communication applications. This may affect the reliability of these devices in their typical high power environments, particularly regarding the power handling characteristics and expected lifetime of the TFR devices, for example. Accordingly, what is needed is a method and apparatus for testing these devices to determine the effects of electromigration thereon, in an effort to determine possible solutions to counter-balancing or avoiding the adverse effects that electromigration might have on overall TFR device performance.
The present invention provides a method and apparatus for determining high power reliability in electronic devices. In an aspect of the method, the frequency of the input power supplied to the electronic device to be tested is swept in a specified range of frequencies, so that the input power is applied to the operational frequency band of the device, including the resonant zero of the device. This technique enables a measure of reliability of the device to be determined, such as the failure time of the electronic device (and hence projected lifetime) that is independent from the frequency of the input power.
Another aspect of the invention provides a thin film resonator device that has been developed based on the results of the test. The TFR device includes a protective layer or film that is deposited on top of an aluminum electrode of the TFR. This protective layer protects the electrode against the effects of electromigration damage. The resultant TFR device with the modified electrode structure delays the time to failure by increasing the critical current density above which the TFR device may be damaged due to electromigration, thereby providing a TFR device having a longer lifetime than, and able to operate at higher operational powers than conventional devices.